The present invention relates generally to pigments comprising high aspect ratio materials encapsulated in a polymer to form beads, and methods of making same. More specifically, the present invention relates to pigments comprising high aspect ratio flake particles encapsulated in a polymer to form substantially spherical beads that are useable for producing plastic products having a colored, sparkling, and/or metallescent or metal-like appearance, and methods of making same.
Current trends in consumer products have created an unprecedented demand for plastics having unique colors and looks. For example, cellular telephones having interchangeable plastic covers are now commonly available. These covers come in a variety of colors and/or special effects, giving consumers control over the aesthetic design of their phones. Many other injection molded plastic products, such as, for example, computers, stereos and other consumer and/or business equipment, automotive interiors, etc., would also benefit from having unique colors and looks.
Plastic products having a sparkling look, a metallescent or metal-like look, a true metal appearance, or any angular metameric appearance are desirable in many instances. Plastics having a sparkling or metallescent look can be created by incorporating free metal flakes into a plastic article at such loading that the individual flakes can be distinguished by the naked eye, resulting in an article with a sparkling or metallescent appearance. Plastics having a true metallic look can be created by using a much higher loading of free metal flakes. The drawback to products incorporating free metallic flakes is that the free flakes have a high aspect ratio, and therefore tend to change orientation at knitlines or other areas of non-uniform flow direction, aligning themselves with the flow field during processing, thereby causing visible flowlines, knitlines or weldlines in the finished product. Such flowlines, knitlines or weldlines produce unattractive streaks in the product. Therefore, there is an increasing demand for sparkling, metallescent and/or metal-looking plastics that cannot be met effectively by incorporating free metallic flakes into plastic articles.
Many desirable pigments and additives for plastics, such as metallic particles, have a high aspect ratio, including, but not limited to, plate-like or flaky particle structures. As previously discussed, during processing such particles tend to orient parallel with the material flow. In some cases, this directionality can be an advantage, such as for improving the flexural modulus of extruded sheet moldings or thin-walled moldings. In other cases, however, this directionality can be disadvantageous, particularly in situations where the parts are complex and a uniform surface appearance is desired. Under such conditions, local variations in flow field direction can produce unacceptable variations in color or reflectivity. Metallic particles/flakes are particularly susceptible to this directionality problem because their reflectivity is high and very directional.
The geometry of the pigment particle is important in determining the optical effects that will be produced. The availability of multiple facets maximizes the reflectivity for a given particle orientation, while a more spherical shape is more hydrodynamically isotropic, thereby favoring the reduction or elimination of flowlines. High aspect ratio particles offer high sparkle efficiency, making them very effective as metallic pigments. However, these particles are also responsible for the appearance of visually objectionable flowlines.
There is presently no good way to ensure that pigments and additives having a high aspect ratio are optically anisotropic and rheologically isotropic when processed into plastic products. Cubes and octahedra seem to satisfy these two conditions, and in fact, existing techniques have attempted to resolve the directionality problem of flake materials by encapsulating the flake materials in a substantially cubic material. While cubic shaped materials are less sensitive to flow-induced orientation than flakes are, and thereby reduce the appearance of flowlines, this technique does not eliminate flowlines. Additionally, producing particles of complicated geometries (i.e., other than spheres and platelets) is nontrivial, and introduces unnecessary filler mass to the final product. As a result, considerable efforts have focused on the investigation of a number of processing solutions to eliminate flowlines. Sequential gating, the use of complicated mold temperature profiles, and agitation of the mold have been found to reduce the appearance of flowlines considerably. However, these methods all require the use of highly specialized equipment, making them extremely uneconomical and impractical. Alternatively, using aluminum beads has been investigated as a possible solution. However, due to the absence of facets or sizeable reflective surfaces in aluminum beads, a reflective metallic look cannot be achieved in the end product.
Japanese Kokai Patent Application No. Hei 11[1999]-279434 describes a metallic pigment prepared by coating a transparent resin on at least one facet of glossy polyhedron particles. Japanese Kokai Patent Application No. Hei 11[1999]-21376 describes composite glittering agents that are cross-linked polymer particles containing glossy particles, resin compositions containing the glittering agents, and molded resin products made therefrom. However, neither of these references addresses the problem of glossy particles delaminating from the composite. Additionally, neither reference addresses the importance of cross-link density to overall physical properties or to the importance of restricting the size of the composite bead particles in obtaining defect free injection molded articles.
Thus, there is a need for systems and methods that ensure that the angular distribution of materials with high aspect ratios (for example, plate-like/flake pigments and additives) in complex flow fields remains optically anisotropic and rheologically isotropic when processed into plastic products. There is also a need for such systems and methods to be less expensive and less highly specialized than existing systems and methods. There is yet a further need for such systems and methods to substantially eliminate the appearance of flowlines. There is still a further need for such systems and methods to encapsulate high aspect ratio materials within a cross-linked or cross-linkable polymer. There is also a need for such systems and methods to provide higher flake-encapsulating bead yields than currently possible. There is yet still a further need for such systems and methods to utilize encapsulated high aspect ratio materials as pigments in injection molded or extruded plastics to create plastic articles substantially free of flowlines and having a sparkling and/or metallescent appearance. There is also a need to address the problem of high aspect ratio materials delaminating from the encapsulant bead during compounding and/or molding operations. Finally, there is a need for such systems and methods to incorporate dyes therein to allow colored, sparkling and/or metallescent appearing products to be created.